It’s unlikely that the playable Nurburgring track will be offered as a download for Gran Turismo 5 Prologue, at least in it’s present state. As this post shows, it’s obviously using assets from Gran Turismo 4, and as such we can’t imagine Polyphony being happy with this being in the general public’s hands.
Top shot is an off-screen photo from GT 5 Prologue, bottom screen is a screenshot from GT 4. Check the link above for more. So, everyone at the track enjoy the special version you’ve got, and the rest of us: forget about it, at least for now.